Such processors generally comprise a bank of Doppler filters tuned to different Doppler frequencies within the unambiguous range established by the pulse-repetition frequency (PRF) of the radar. The filters are connected to the signal extractor of the radar by evaluation circuitry usually including a threshold circuit for the output of each filter. The operation of processors of the MTI (moving-target indicator) or the MTD type is well known in the art; reference may be made to the following publications:
[1] M. I. SKOLNIK, "Introduction to Radar Systems", 2.sup.nd Edition, McGraw-Hill Book Co., New York, 1980.
[2] D. C. SCHLEHER, "MTI Radar", Artech House, 1978.
[3] C. E. MUEHE, "Moving-Target Detector, an Improved Signal Processor", AGARD Conference Proceedings No. 195, 14-17 June 1976, pp. 14-1 to 14-10.
[4] R. M. O'DONNELL and C. E. MUEHE, "Automated Tracking for Aircraft Surveillance Radar Systems", IEEE Transactions, Vol. AES 15, No. 4, July 1979, pp. 508-516.
[5] D. KARP and J. R. ANDERSON, "MTD-II Summary Report", MIT Lincoln Laboratory, ATC-95, June 1, 1981.
[6] J. R. ANDERSON and D. KARP, "Evaluation of the MTD in a High-Clutter Environment", IEEE International Radar Conference, 1980, pp. 219-224.
[7] J. W. TAYLOR, Jr., "Sacrifices in Radar Clutter Suppression Due to Compromises in Implementation of Digital Doppler Filters", Radar 82 Conference, London, 18-20 Oct. 1982, pp. 46-50.
[8] E. CRISTOFALO and G. GALATI, "On the Azimuthal Accuracy of an FFT-Based Signal Processor for Air Traffic Control Radars", Rivista Tecnica Selenia, Vol. 7, No. 1-1980, pp. 8-16.
The several threshold circuits may include fixed-threshold or adaptive-threshold comparators. An adaptive threshold, known as Autogate, is described in the following publication written by one of us in a periodical issued by our assignee:
[9] G. GALATI, "The `Autogate` Circuit in Radar Detection: Analysis and Comparison of Alternative Configurations", Rivista Tecnica Selenia, Vol. 1, No. 3-1973, pp. 19-29.
A drawback of conventional MTD processors of the type referred to lies in the inalterability of their Doppler filters, preventing an optimization of their characteristics in the presence of difference environmental conditions such as the absence or presence of ground-clutter and rain-clutter interference.
Similarly, the presence of an adaptive threshold such as the Autogate is useful in the presence of heavy rain but is an unneeded encumbrance otherwise, resulting in a loss of sensitivity to useful targets to be detected.